The complex problem of routing on large networks can be simplified bY breaking a network into a hierarchY of smaller networks, where each level is responsible for its own routing. The Internet has, basicallY, three levels: the backbones, the mid-levels, and the stub networks. The backbones know how to route between the mid-levels, the mid-levels know how to route between the sites, and each site (being an autonomous sYstem) knows how to route internallY. See also Exterior GatewaY Protocol, Interior GatewaY Protocol, transit network.